1. Field of the Invention
This invention relates to the use of fibers (e.g., wicking fibers) in electrochemical batteries, electrochemical double layer capacitors, and asymmetrical capacitors.
2. Related Art
It is known to provide electrical power storage devices, such as electrochemical batteries and capacitors, for use in vehicles such as automobiles. For example, lead-acid batteries have been used in starting, lighting, and ignition (“SLI”) applications.
Two of the most common electrical power storage devices are batteries and capacitors. Conventional lead-acid batteries electrodes are formed by creating a lead paste that is applied to a substrate (e.g., a grid, plate, or screen) that may also ad, as a charge collector. As the lead paste dries, open pores are formed within the lead paste where battery electrolyte may enter increasing the reactive area of the grid and increasing its charge capacity. However, excessive porosity reduces the electrode's structural integrity. In addition, because conventional electrodes have limited porosity, a significant amount of active material is inaccessible to the electrolyte and is underutilized or essentially wasted because it is not available for reaction. Typically, about half of the lead in conventional lead-acid electrodes is unusable or goes unused. Over its life, a battery may be charged and discharged multiple times, which can also degrade the electrode as the reduction-oxidation reactions that supply current are repeatedly reversed. Over time, sections of the electrode can become electrically disconnected from the rest of the electrode. The electrode's structural integrity also deteriorates over time. To hold the electrode material in place, a scrim layer (e.g., a fiber mesh) may be used. The scrim layer mat may be placed on the charge collector prior to applying the active material paste and/or placed over the paste after it is applied. The scrim layer may help hold the electrode together, but it does not improve porosity or increase reactivity.
Capacitors store power in the form of an electric field between two conductors. Typical capacitors use stacks of thin plates (alternating capacitor plates and dielectric) or rolls of thin sheets (alternating capacitor and dielectric sheets rolled together). Energy is commonly stored on adjacent plates or sheets, separated by dielectric material, in the form of electrical charges of equal magnitude and opposite polarity. In typical capacitors, current flows from the capacitor surface throughout the entire capacitor plate, requiring the plates to be conductive to reduce resistance loss and to be sufficiently thick to not overheat and melt. Such requirements impose undesirable limits on the capacitor's power storage to weight ratio. Capacitance (i.e., the amount of charge stored on each plate) is proportional to plate surface area and inversely proportional to the distance between plates. Thus, increasing a capacitor's ability to store energy often requires increasing plate size and/or decreasing the distance between the plates. However, increasing the plate size increases resistance and overheating problems and decreasing plate separation increases the risk of charge passing directly between the plates (i.e., a short circuit), burning them out and rendering the capacitor incapable of holding a charge.
Electrochemical double layer capacitors (“EDLC”) are power storage devices capable of storing more energy per unit weight and unit volume than traditional electrostatic capacitors. Moreover, FDIC can typically deliver stored energy at a higher power rating than conventional rechargeable batteries. Conventional EDLC use carbon as the active material in the electrodes. Conventional EDLC consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and electrodes are infused with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator, but prevents electrical current from shorting the cell. A current collecting grid is coupled to the back of each of the electrodes. EDLC store electrostatic energy in a polarized liquid layer that forms when a potential exists between two electrodes immersed in an electrolyte. When electrical potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface by polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes. No reduction-oxidation reactions are involved in the charge storage mechanism.
Asymmetric electrochemical capacitors use a battery electrode for one of the electrodes. The battery electrode has a large capacity in comparison to the carbon electrode, so that its voltage does not change significantly with charge. This allows a higher overall cell voltage. Examples of asymmetrical capacitors materials include PbO2 with carbon and NiOOH with carbon.